In general, a cathode of a high luminance discharge lamp that receives a high input or other lamps contains, as an additive, an emitter to facilitate electron emission. For example, Japanese Patent Application Laid-Open Publication No. 2012-15008 (Patent Literature Document 1) discloses a cathode for use in a discharge lamp, which contains a thorium oxide as an emitter.
However, thorium is a radioactive substance, and use of thorium is restricted (regulated) by laws. Thus, handling and managing thorium need careful attentions, and there is a demand for an alternative to thorium.
One of the alternatives to thorium is a rare earth element, another alternative is a compound of rare earth element(s). The rare earth element has a low work function (in general, the work function indicates an energy needed for an electron to jump out of a substance), and is excellent in electron emission. Thus, the rare earth element is expected to be used in place of thorium.
Japanese Patent Application Laid-Open Publication No. 2005-519435 (Patent Literature Document 2) discloses a cathode for use in a discharge lamp, and the material of the cathode (tungsten) additionally contains, as an emitter, lanthanum oxide (La2O3), hafnium oxide (HfO2), zirconium oxide (ZrO2) or the like.
However, the rare earth oxide such as lanthanum oxide (La2O3) has a higher vaporization pressure than thorium oxide (ThO2), and is relatively easy to vaporize. Accordingly, if the rare earth oxide is used as the emitter to be contained in the cathode, in place of thorium oxide, then the rare earth oxide excessively vaporizes and is depleted quickly. When the emitter is depleted, the cathode loses the electron emission function. Then, flicker occurs, and the life of the lamp is reduced.
The emitter that contributes to the electron emission is only present at (in) a front end of the cathode. Another reason for the depletion of the emitter is because the emitter is not quickly conveyed to the front end from a rear portion of the cathode. In reality, therefore, the discharge lamp that uses a substance other than thorium oxide as the emitter material still has a problem, i.e., the light emission becomes unstable quickly. In particular, when the discharge lamp is a high-input lamp that receives an electricity of 1 kW or more, then the vaporization of the rare earth element and barium-based substance causes the discharge lamp to emit light in a significantly unstable manner.
Japanese Patent Application Laid-Open Publication No. 2002-141018 (Patent Literature Document 3) discloses a configuration of a cathode that uses an alkaline earth metal (oxide) as the emitter material. FIG. 19 of the accompanying drawings shows the configuration of the cathode. An easy electron emission part 81 is embedded in a cathode 80. An alkaline earth metal oxide is contained, as an emitter, in the easy electron emission part 81. The easy electron emission part 81 is exposed at the front end of the cathode.
Even in this configuration, the alkaline earth metal oxide, which is the emitter, is subjected to an arc. Thus, vaporization of the emitter is further accelerated. This is similar to the electrode disclosed in the above-mentioned Patent Literature Document 2. As a result, similar problems occur. Specifically, the emitter is depleted quickly in particular at the front end of the cathode, and the cathode loses the electron emission function. Then, the flicker is generated. This reduces the life of the lamp.